An 18 kDa protein (MIRL) from normal erythrocytes (E) that inhibits complement-mediated hemolysis of the E of PNH has recently been purified in our laboratory. Analysis by Western blot using anti-MIRL showed that PNH E are deficient in MIRL. These studies indicate that that the abnormal susceptibility of PNH E to complement-mediated lysis is causally related to decreased expression of MIRL. By virtue of its functional activity and molecular weight, MIRL appears to be distinct from other previously described E membrane inhibitors of complement. Herein, we propose to fully characterize MIRL. Using immunochemical techniques, quantitative and qualitative analysis of the relationship between MIRL and the three phenotypes of PNH E will be characterized. In analogous experiments, the expression of MIRL on other hematopoietic cells from normal volunteers and patients with PNH will be analyzed. Other proteins that are absent from PNH cells have been shown to be anchored to the cell-surface by a glycosylphosphatidylinositol moiety. A phosphatidylinositol-specific phospholipase C will be use d to determine if MIRL is also linked by a glycolipid tail. The mechanism by which MIRL inhibits hemolysis will be investigated by determining the complement components. Finally, we propose to clone the MIRL gene and study the expression of MIRL mRNA. The results of the proposed studies should help delineate the physiological function of MIRL and provide new insights into the pathophysiology of PNH.